Abstract

Positron annihilation lifetime spectroscopy (PALS) is used to study the ionic liquid 1-methyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide in the temperature range between 150 and 320 K. The positron decay spectra are analyzed using the routine LifeTime-9.0 and the size distribution of local free volumes (subnanometer-size holes) is calculated. This distribution is in good agreement with Fürth’s classical hole theory of liquids when taking into account Fürth’s hole coalescence hypothesis. During cooling, the liquid sample remains in a supercooled, amorphous state and shows the glass transition in the ortho-positronium lifetime at 187 K. The mean hole volume varies between at 150 K and at 265–300 K. From a comparison with the macroscopic volume, the hole density is estimated to be constant at corresponding to at 265 K. The hole free volume fraction varies from 0.023 at 185 K to 0.073 at and can be estimated to be 0.17 at 430 K. It is shown that the viscosity follows perfectly the Cohen–Turnbull free volume theory when using the free volume determined here. The heating run clearly shows crystallization at 200 K by an abrupt decrease in the mean and standard deviation of the lifetime distribution and an increase in the intensity . The parameters of the second lifetime component and behave parallel to the parameters, which also shows the positron’s response to structural changes. During melting at 253 K, all lifetime parameters recover to the initial values of the liquid. An abrupt decrease in is attributed to the solvation of and particles. Different possible interpretations of the lifetime in the crystalline state are briefly discussed.

Received 09 February 2010Accepted 17 August 2010Published online 24 September 2010

Acknowledgments:

Dirk Pfefferkorn from the Institute of Chemistry of the Martin-Luther-University Halle-Wittenberg is acknowledged for performing the PVT experiments. We thank also Jerzy Kansy, Institute of Materials Science, Silesian University, Katowice, for supplying the analyzing routine LifeTime (LT) and the simulation routine simLT.